1. Field of the Invention
This invention relates to an improved chemical milling solution and method for milling, etching or pickling metal products therewith. More particularly, the invention relates to a bath composition and method for milling or pickling titanium workpieces, such as forgings or the like.
2. Technology Review
As used herein, the term "milling" shall mean the selective and controlled removal (or corrosion) of metal (or metal oxides) from a part or object by chemical milling, etching and/or pickling. Most milling procedures form metal product of a desired thickness and/or configuration by removing metal from treated workpieces and imparting greater weight savings to aerospace parts or the like. Milling operations are typically performed after particular metal parts have been formed by casting, forging, extrusion or rolling, then heat treated. Milling is also used to make shapes which cannot otherwise be machined by conventional chipmaking techniques, or which can only be machined by known methods at unreasonably high cost. For many parts, masking of certain areas is done to prevent their exposure to a corrosive milling solution.
As used for the description of this invention, "milling" shall also include metal etching, the controlled removal of metal for dimensional and shape control, and metal cleaning or pickling, i.e., the removal of embrittled oxidized surfaces. For titanium alloys, oxidized surfaces are sometimes referred to as alpha-case. Such surfaces typically result from elevated temperature exposure in the manufacturing process, i.e., casting, rolling, extrusion, forging or the like.
Any chemically-dissolvable metal may be subjected to treatment by the aforementioned milling practices. Alloys of aluminum, beryllium, magnesium, titanium and various steels are the most commonly milled metal products. Refractory metals such as molybdenum, tungsten, niobium (columbium) and zirconium may also be chemically etched in a similar manner. The workpieces treated by milling (i.e. chemical, etching and/or pickling) need not be limited in size, provided a large enough bath of solution can be maintained. Milled parts may be cast, forged, extruded or rolled. Their end shapes may be flat, tubular or in any of the complex configurations required by today's manufacturers of aerospace and other parts.
The first chemical milling practices are believed to have occurred around 2500 B.C., when ancient Egyptians used citric acid to etch copper jewelry. Current industrial milling practices can be traced back to the methods set forth in Sanz U.S. Pat. No. 2,739,047. Numerous evolutions to milling solutions have occurred since modern milling procedures were patented over 35 years ago. Many of these solution developments depended on the particular metal alloy being milled.
High carbon chromium steel may be chemically etched in a solution containing 300-500 ml/1 HCl; 50-150 ml/1 HNO.sub.3 ; 20-40 g/l NaNO.sub.3 ; 10-30 g/1 ammonium persulfate and 20-50 ml/1 H.sub.2 O.sub.2 according to Russian Patent No. 505,750.
Aluminum foil is cleaned with a water solution comprising 1-5% ammonium persulfate, 1-5% sodium chlorate and 1-3% of a surfactant selected from dioctyl sodium sulfosuccinate and dodecyl benzene sulfonate in Flowers U.S. Pat. No. 3,954,498. Russell et al U.S. Pat. No. 4,337,114 dissolves nodular copper from aluminum foil surfaces using a 0.1-2.0M solution of ammonium persulfate.
In Hanazono et al U.S. Pat. No. 3,905,883, thin films of permalloy, iron, nickel, cobalt and copper are etched with an electrolyte comprising 0.1-2.5 mol/1 ammonium persulfate and 0.02-10 mol/1 nitric acid.
Copper etching presents a different set of problems. Matsumoto et al U.S. Pat. No. 3,936,332, for instance, teaches etching copper with a solution containing at least 5% by weight peroxysulfate, at least 50 parts per million (ppm) of a diazine compound and from 5 to 2000 ppm of a halogen compound selected from: HF; HBr; HCl; salts thereof; oxides of Fl, Cl, Br and I; and oxygen-containing acids of Cl, Br and I; among other possibilities. In Matsumoto et al U.S. Pat. No. 3,939,089, copper is etched with a solution of peroxysulfate, purine and a halogen. Pryor et al U.S. Pat. No. 4,725,374 etches copper with a solution of 0.5-6.0 N peroxydisulfuric acid and 10-500 ppm chloride or fluoride. A process for etching copper with substantially the same solution(s) as above is claimed in related Pryor et al U.S. Pat. No. 4,973,380.
For titanium and titanium-based alloys, Chen U.S. Pat. No. 4,900,398 claims a milling method which uses an aqueous solution consisting essentially of 1-5% hydrofluoric acid, 1.5-4% chlorate ion and, optionally, up to 20% of an acid selected from the group consisting of H.sub.2 SO.sub.4, HCl and HO.sub.3.
Kremer et al. U.S. Pat. No. 4,314,876 discloses a milling solution consisting essentially of: 3-10 wt.% ammonium bifluoride; 5-15 wt.% nitric acid, or its equivalent as ammonium nitrate, sodium nitrate or potassium nitrate; 2-25 wt.% hydrochloric acid when ammonium nitrate, sodium nitrate or potassium nitrate is used as the nitrate source; up to 1 wt.% wetting agent; and 92-49 wt.% water. According to the examples, this solution removes Ti metal at rates ranging from 0.000027 to 0.00074 mils/side/minute.
In Coggins et al. U.S. Pat. No. 4,116,755, there is claimed a method and composition for milling titanium without excessive hydrogen absorption. The composition comprises, per liter of solution: 126-700 grams of pure nitric acid or its equivalent; the equivalent of 8.8-176.1 grams of pure hydrofluoric acid; at least 10 grams of a carbonic acid derivative; and at least 1.5 grams of a monocarboxylic acid derivative containing alkali metal ions.
Coggins et al. U.S. Pat. No. 3,944,496 claims a milling composition for titanium and other refractory metals which comprises: 210-630 grams of pure nitric acid; 98-440 grams of pure phosphoric acid or its equivalent; 61-88 grams of pure hydrofluoric acid, or its fluoride-producing equivalent; and a carbonic acid derivative equivalent to 15 grams or more of carbamide.
In Roni U.S. Pat. No. 3,844,859, an improved method for milling titanium includes immersing metal in an aqueous fluid containing: a sufficient amount of hydrofluoric acid for effecting an etch rate of 4-15 mils/side/minute; a sufficient amount of dodecylbenzene sulfonic acid and linear alkyl sulfonic acid for keeping the surface tension of this fluid between 28-60 dynes/cm; and 0.2-1.2 wt.% nitric acid. Between 0.07 and 2.9 wt.% ammonium bifluoride may be added to this solution for reducing channeling and ridging in the fillet areas of a vertically-milled part.
Gumbelevicius U.S. Pat. No. 3,788,914 employs a titanium milling solution which contains, per liter of solution: 126-682 grams of nitric acid; the equivalent of 8.8-176.1 grams of pure hydrofluoric acid; and at least 10 grams of a carbonic acid derivative selected from carbamide, urea nitrate, urea oxalate and semi-carbazide.
Kreml U.S. Pat. No. 3,666,580 discloses a milling solution comprising 2-10 vol.% hydrofluoric acid and 1-10 vol.% hydrochloric acid, with a remainder of water. This solution is maintained at a temperature between 65.degree.-140.degree. F. for the milling of titanium metal parts therein.
The milling composition of Snyder et al. U.S. Pat. No. 2,981,610 contains: 0.1-4.7 molar nitrate; 0.1-2.2 molar chloride; 0.25-5.3 molar fluoride; at least 0.22 normal acetate; and a hydrogen ion concentration of 2.8-10.7 molar.
Russian Patent No. 1,294,872 removes oxide film from the surface of titanium articles by etching them for 20-25 minutes in molten ammonium persulfate heated to 300.degree.-340.degree. C., washing the articles with water, holding them for 3-5 minutes in a 30-40% solution of H2S04 heated to 60.degree.-70.degree. C., followed by water washing and air drying.
Current practices for chemically milling, etching and pickling titanium workpieces employ baths of hydrofluoric acid and nitric acid in various concentrations. Hydrofluoric acid poses risks to the health of its day-to-day handlers, however. Any process employing HF poses yet another major risk in the event said additive is accidentally released into the environment. Hydrofluoric acid is being considered for greater Federal regulation because of such concerns. Nitric acids, on the other hand, release visible fumes of toxic NO.sub.x during standard milling operations. Emission source locations are also under increasing regulatory pressure to reduce or eliminate such toxins from the workplace. Although hydrofluosilicic acid (H.sub.2 SiF.sub.6) has been proposed as an HF substitute, this liquid is also hazardous and quite volatile.